You cannot use I haven't known that here. The present perfect describes a present state which arises out of a prior eventuality, and you are implicitly saying that your present state is that you do know that—which is not a state that can arise out of your previous ignorance. (It can emerge from your previous ignorance, but it can only arise out of learning it.)
You may use I didn't know that, with or without until you mentioned it. This states that in the past you were in a state of ignorance, a state which ended at the point when your addressee mentioned 'it'. Until marks the end of a state, as explained here.
You are not obliged to include anything like until you mentioned it in order to make it clear that now you do know that—the discourse situation takes care of that—it serves only to make clear that it was your addressee's statement which dispelled your ignorance, not some other past event.
You may also use I hadn't known that, again with or without until you mentioned it. The past perfect does not necessarily describe a state arising out of a prior eventuality, because the past tense-domain does not have the contrast between simple past and present perfect which exists in the present domain—the past perfect serves for both. Here the past perfect acts as a “past-in-past”, analogous to the present-tense-domain simple past, so it describes a prior state of ignorance which ended at your past reference time, the time explicitly named in the until clause.
However, as you know, FumbleFingers' Perfect Truism instructs you that if you do not need a past perfect you should not use it. In this case there is no evident reason why a past perfect should be needed; consequently the best choice here is the simple past:
I didn't know that (until you mentioned it).
The way you have it is the more natural way to say it (and indeed this expression is often said with exactly these words in English). The English simple present tense is used to describe an ongoing situation or a general truth, like this one. This is true even if the event that you are interpreting took place in the past. Example:
Cheryl: I broke my leg last year.
Alice: Well, what doesn't kill you makes you stronger.
Cheryl: Actually, it made me a lot weaker, because I could only walk on one leg.
You could make it past tense if you were describing a specific event, but that would be a little weird, since the present tense version of this specific proverb is a well-known phrase:
Cheryl: I broke my leg last year.
Alice: Since that didn't kill you, that must have made you stronger.
Cheryl: Our friendship is over.
Finally, you can use the past tense if the maxim is no longer relevant.
Alice: When I was young, what didn't kill me made me stronger.
Best Answer
The difference between your two examples is the difference between the perfect and preterite(simple past) tenses.
The main semantic difference is, as detailed by the following article, one of time-frame.
If you swap their examples for yours it all fits into place; humans didn't walk infers that at a specific time in the past humans did not walk on Neptune, while have not walked denotes that from the start of humanity to now humans have not walked on Neptune.
In other words, the present perfect also implies some temporal connection to the present, now.
In contrast, the preterite refers to a specific designated time-frame e.g. last week humans did not walk on Neptune.